1. Field of the Invention
The present invention relates to a technique of dynamically adjusting the counter electrode voltage of a liquid crystal panel used for an image display apparatus according to illumination light control.
2. Description of the Related Art
One proposed technique applicable to a projector or another image display apparatus controls the light level of a lighting device to be suitable for input image data, so as to regulate the luminance of a resulting displayed image and improve the picture quality of the displayed image (see, for example, Japanese Patent Laid-Open Gazette No. 2004-45634).
Liquid crystal panels are widely used in the image display apparatus as the electro-optic device for displaying an image represented by input image data. The liquid crystal panel changes a voltage applied to each pixel electrode specific for each pixel (pixel electrode voltage) relative to a voltage applied to a counter electrode common to respective pixels (counter electrode voltage), based on image data of each pixel. The transmittance of illumination light for irradiating each pixel is regulated in response to the variation in pixel electrode voltage. The liquid crystal panel outputs image light with the regulated light transmittance, which is focused on a screen or another equivalent device to display an image.
The optimum level of the reference counter electrode voltage is varied with a change in amount of illumination light for irradiating the liquid crystal panel. As is known in the art, a significant deviation of the counter electrode voltage from the optimum level causes a trouble, such as flicker or burn-in, to deteriorate the image quality. One conventionally known technique adopted in the image display apparatus with the liquid crystal panel sets the counter electrode voltage to the optimum level before shipment and adjusts the counter electrode voltage to the varying optimum level with a time change of illumination light (see, for example, Japanese Patent Laid-Open Gazette No. 2005-221569).
The light control of the lighting device according to the input image data, however, has a problem of varying the optimum level of the counter electrode voltage with a dynamic change in amount of illumination light for irradiating the liquid crystal panel.